The present invention relates to measurement probes and more particularly to electrical test probes for coupling electrical signals to testing equipment from a device under test.
A variety of measurement probes, such as low and high voltage probes, current probes, temperature probes, and the like, have been developed for monitoring functions in devices under test. These probes have a signal input head connected to a termination output coupler via a flexible transmission line, or cable. The termination output coupler is connected to an input of an oscilloscope or other electrical test instrument and the signal input head is used to probe the device under test.
Low voltage probes are generally used for monitoring voltages in electronic circuits. To reduce electromagnetic interference, EMI, and prevent the degrading of the electrical signal being monitored, low voltage probes have a probe tip coaxially disposed within one end of a elongate cylindrical outer conductor with one end of the probe tip extending beyond the end of the outer conductor for probing the device under test. An insulating plug is used to secure the probe tip in the cylindrical outer conductor. Passive or active circuitry mounted on a substrate is disposed within the elongate cylindrical outer conductor and is electrically coupled to the other end of the probe tip. The transmission line is electrically connected to the substrate to couple the electrical signal from the signal input head to the termination output coupler, which is connected to the input of the test instrument. Insulating material surrounds a substantial portion of the elongate cylindrical outer conductor. A portion of the outer conductor is exposed near the probe tip to allow the placement of various probing adapters, such as a BNC type connector on the probe. A representative low voltage probe having the above characteristics is the P6106A Passive Voltage Probe, manufactured and sold by Tektronix, Inc., Beaverton, Oregon. A BNC type probe adapter for use with the P6106A probe is manufactured and sold by Tektronix, Inc., under part No. 013-0084-02.
The BNC probe adapter has a BNC connector at one end and a cylindrical metal tube extending from the other end. The metal tube has a first inner diameter providing clearance for the insulating material surrounding the cylindrical outer conductor of the probe. A second inner diameter, proximate the BNC connector, is slightly smaller than the cylindrical outer conductor of the probe and has slots formed parallel to the axis of the tube creating fingers for gripping the end of the cylindrical outer conductor. The BNC connector has a central conductor coaxially disposed and electrically insulated from an outer conductor. The outer conductor of the BNC connector is electrically connected to the cylindrical metal tube. The probe tip of the probe mates with the central conductor of the BNC connector and the cylindrical metal tube engages the cylindrical outer conductor of the probe to provide a coaxial signal path for signals coupled through the BNC connector to the probe.
Voltage measurement probes have a voltage rating to warn a user that potential life threatening electrical shock can occur if the probe is used beyond the specified rating. Arcing between the probe tip and the outer conductor of the probe is possible if the voltage rating of the probe is exceeded. Insulation standards have been set by Underwriters Laboratories, UL, for electrical and electronic measuring and test equipment (UL1244) that establishes minimum distances between conductive elements and between conductive elements and the user for particular voltage ratings. For example, electrical equipment having a peak AC voltage rating of between 212-315 volts must have a minimum distance separation through air of 0.140 inches and an over surface separation of 0.180 inches between conductive elements and between any exposed conductive element and a user. Generally, low voltage probes are single insulated devices. The UL standard for single insulation is two times the peak AC voltage rating of the probe plus 1,414 peak AC volts during 60 Hz high voltage testing. The standard for double insulated probes is 4 times the peak AC voltage rating of the probe plus 2,878 peak AC volts. As the voltage level rating for a probe increase so does the minimum distance separation standards. This causes high voltage probes to be much larger than low voltage probes.
In most applications, voltage probing is performed with the voltage signal being referenced to a common voltage ground. The common voltage ground comes from the test instrument and is coupled to the signal input head through the termination output connector and the transmission line of the probe. Additionally, an electrical connection is provided at the signal input head for connecting the outer conductor of the signal input head to the common voltage ground of the device under test. Specially designed test equipment can perform floating measurements where the common voltage is no longer grounded, but can float to a non-ground voltage potential. In this case, the common voltage comes from the device under test and not the test instrument. An electrical connection is provided at the signal input head for connecting the outer conductor of the probe to a voltage source. For high voltage floating measurements, user safety requires that both the signal and the outer conductor of the measurement probe be shielded. In addition, UL standards require double insulation for probes making floating measurements.
Conventional low voltage probes do not meet the strict safety requirements for floating measurements and floating measurement probes specifically designed for high voltage floating measurements cannot be used with probe adapters for common voltage ground measurements. What is needed is a measurement probe that is usable as a floating measurement probe and a low voltage probe with probe adapters.